Hello. How can it be possible for an organism to
survive living a intermediate state of evolution.
Like bats, was there a point in their
evolutionary process where they had small wings? I
hope you understand my question, because it's the
best way I could write it. Thanks.

Question Date: 2003-07-03

Answer 1:

That's an excellent question, and you're
thinking along the right lines to ask it. To use
your example, how would some ancestral bat
species with smaller wings "know" that it had to
evolve a little more to become an evolutionarily
successful bat species with longer wings? And
how would the species survive if its small
wings were less than evolutionarily
successful? Get ready, because my answer is
long.

There are two ways to think about evolutionary
change. You can picture it as a gradual process,
where a species undergoes small changes very
slowly, taking enormously long periods of time to
evolve into a new species. This is sort of the
classical view of evolution. (Remember that
individual animals don't evolve, populations and
species do, based on which individuals reproduce
better and what traits they pass on to future
generations.) If this gradual change was what
happened with your bats, it would have to be
because the intermediate forms (those with shorter
wings), even though they were less successful than
current bats with longer wings, were still more
successful than previous forms (probably some
mouse-like ancestor without wings). So, longer
wings lead to better reproductive success, and
EVEN longer wings lead to EVEN better reproductive
success.

If you follow your question backward through
evolutionary time, you would next have to ask
why it was advantageous for wings to evolve in
the first place? In all cases, according to
this gradualist view of evolution, no trait
will persist through evolutionary time unless it
is in some way advantageous to an animal's
reproductive success. Animals don't go
through evolutionarily disadvantageous forms in
order to get to the advantageous forms.

There's another way to look at evolution,
though. You could picture evolution doing very
little for long periods of time, and then bam!--a
sudden, huge change that results in a new species
in a very short time. This second viewpoint is
often called "punctuated equilibrium," and
more and more scientists think that it is a better
description of the way things work.

According to the punctuated equilibrium
viewpoint, tiny evolutionary changes actually
happen very quickly. To switch examples, imagine a
species of fish that attracts mates with its
bright colors, and you suddenly make their pond
murky so that it's hard for them to see each
other. In a surprisingly small number of
generations, the fish population will have adapted
to the new conditions, thus evolving. (Remember:
this is not a conscious "decision" on their part,
it's just a matter of which individual fish happen
to reproduce better in the new environment.)

Different traits in the population that were
sort of neutral in terms of reproductive success
may suddenly become advantageous. These traits
would suddenly become more common in the
population--that's evolutionary change. The
fish population may become even brighter, or the
fish may start attracting each other by smell
instead of color. Either way, it will happen very
quickly, and it will depend entirely on the random
collection of traits in the population and which
traits lead to increased reproductive success
under different environmental conditions. The
speed of this change is the "punctuated" part of
punctuated equilibrium.

The "equilibrium" part refers to the fact that,
before you showed up and muddied their water, the
fishes' color was in perfect balance with the
clarity of the water. Maybe if it rained a lot for
a year or two and runoff made the water just a
little murkier, brighter fish would breed better
and the fish population would get just a little
brighter overall. In drier years, then, maybe
duller fish would breed better and the fish
population would get a little duller again. This
is an equilibrium with their environment.

Most of us would probably never notice such
tiny evolutionary changes happening right in front
of us because, despite its ups and downs, the
water has an average level of murkiness, so the
fish population has an average level of
brightness, and we think nothing is happening.
Then, when something big suddenly happens (maybe
an earthquake that makes the pond permanently
muddier) we notice a big, sudden, evolutionary
change in the fish population: they now attract
each other with smell instead of color. This is an
example of punctuated equilibrium without any
intermediates between the old form and the new
form.

So, I would say there are two possible answers
to your question. Either intermediate forms are
themselves advantageous (even if less so then
current forms), or intermediate forms are rare,
because evolutionary changes are big and fast
instead of slow and gradual. The fossil record is
pretty sketchy, by the way--I think sometimes
people see fossils of mice with big webbed feet
and later fossils of bats, and they assume that
there must have been short-winged bats in between.
Maybe there weren't, though.